The nozzle block in a high pressure cylinder in a steam turbine is normally forged from an alloy, such as a 12 percent chromium steel, and is formed from a plurality of individual arc segments having a passage therethrough. A series of radially positioned airfoil vanes are situated in the passageway, to permit steam passage therethrough, which airfoil vanes are initially electric discharge machined from the forging. The vanes each have a leading edge (facing the steam inlet, or upstream) and a trailing edge (facing the outlet, or downstream).
The forged and electric discharge machined vanes sometimes become damaged during use by erosion caused by solid particles in the steam and/or by vibration which leads to fatigue damage (chipping). This damage results in metal being removed from the vanes and they then become thin and fragile. Such weakened vanes are subject to cracking, bending and subsequent breakage.
Attempts have been made to repair the vanes by weld repair of the vanes, to replace the damaged section with subsequent diffusion coating of the vanes, but were not successful. This is probably because of fatigue strength loss due to the welding and further fatigue strength loss due to the diffusion coating.
Since the machined nozzle block is large and expensive, damaged service exposed vanes are normally weld repaired rather than replacing the complete block. Conventional repair processes replace lost metal from a damaged section of a vane by depositing an alloy weld material, such as 12 percent chromium alloy weld material. Such a repair process presents problems including: (1) the extent of welding the trailing edge of the airfoil vane completely creates permanent defects, such as a loss of fatigue strength in the weld repaired portion, and possible inclusions, imperfections and underlying cracks can be undetectable; (2) hand welding and grinding can only restore the airfoil vane to 80-90 percent of its originally manufactured condition, since it is unrealistic to expect 100 percent as time constraints to get the nozzle block back into service as quickly as possible will not permit the period of time needed to restore the airfoil vane completely to original condition; (3) the repair process is very slow, during which time the turbine is out of operation; and (4) such a weld-repaired nozzle block is less rugged than the original forged metal, because the weld metal in presence of weld imperfections has less erosion resistance than a forged material, especially at the critically-stressed trailing edge of the vane and weld, because such metal contains a high content of delta ferrite, which reduces creep and fatigue strength of the airfoil vanes.
Some procedures for repairing turbine components have proposed the use of new sections of the component that are used to replace damaged sections. In U.S. Pat. No. 4,832,252, for example, a method of repairing turbine blades is disclosed that welds or brazes an insert of a material compatible with the material from which the blade is made, with replacement of a damaged portion of the blade by the insert. The insert is hardened at the outer end, while the inner end that is secured to the blade is substantially unaffected by the hardening. The insert is secured to the blade by welding or brazing, while using a weld or braze material of a relatively ductile nature to provide a cushion between the blade and the insert.